1. Field of the Invention
This invention relates to a light emitting semiconductor device, such as semiconductor laser device and a light emitting diode. More particularly, the invention relates to a light emitting semiconductor device which is suitable for high-speed modulation.
2. Related Art Statement
With the progress of communication technology, there is an increasing demand for the development of a light emitting semiconductor device whose output can be modulated at a high speed.
To modulate the radiated output from a light emitting semiconductor device, a direct modulation method has been widely used heretofore, which method modulates by applying a modulating signal to an injection current that is a source of light emission from the light emitting device. This direct modulation method controls the amount of light emission or amplification gain factor for light waves by regulating the number of electrons and holes in an active layer. With such control, the time necessary for switching is bound to be the recombination lifetime of the electrons and holes injected into the active layer, so that the method has a shortcoming in that the switching time is limited.
There is also a Q switching method for modulation by Q switching and a mode locking method for generating a series of light pulses by producing a special operational mode in which the phase relationship among resonant modes is fixed. However, the Q switching method and the mode locking method have shortcomings in that they are complicated and equipment for these methods is difficult to make and costly. The Q switching method requires a light modulator in a resonator, and the mode locking method requires a resonator of complicated structure which is to be mounted on the outside.
To overcome such shortcomings of the conventional methods for modulation of radiated output from a light emitting semiconductor device, the inventors disclosed a light emitting semiconductor device of voltage control type in their Japanese Patent Laid-open Publication No. 60-1,874(1985). FIG. 6 is a schematic illustration of the essential portion of such a light emitting semiconductor device of the voltage control type which the inventor proposed previously. An active layer 1 has insulation claddings 2 and 3 formed on the top and bottom surfaces thereof respectively. A first control electrode 4 is attached to the insulation cladding 2, while a second control electrode 5 is attached to the other insulation cladding 3. A p-type injection region 6 and an n-type injection region 7 are attached to opposite sidewalls of the active layer 1. The p-type injection region 6 injects holes into the active layer 1, while the n-type injection region 7 injects electrons into the active layer 1. The injected electrons and holes recombine within active layer 1 and emit light therein.
When a positive control pulse voltage and a negative control pulse voltage as shown in FIG. 7A are applied to the first and second control electrodes 4 and 5 respectively, the injected electrons in the active layer 1 are captured in the active layer 1 on the side of the first control electrode 4, while the injected holes are also captured therein on the side of the second control electrode 5. Thus, a kind of polarization occurs. If the forbidden gap of the active layer 1 is selected to be smaller than the forbidden gaps of the insulation claddings 2 and 3, the injected electrons and holes are prevented from being extracted to the insulation claddings 2 and 3 by the electric field there. Accordingly, very quick modulation of the radiated output is made possible, without being restricted by their recombination lifetime, by using the control pulse voltage applied to the first and second control electrodes 4 and 5 as modulating signals.
The above light emitting semiconductor device of voltage control type, which was previously proposed by the inventors, has an outstanding merit in that its modulating speed of the radiated output is not restricted by the recombination life time of the injected electrons and holes, but it has the following limitations. The p-type injection region 6 and the n-type injection region 7 maintain their constant-rate injection of holes and electrons even when the control pulse voltages are applied (non-emitting periods), and the number of the electrons and holes increases gradually. Accordingly, an undesirable increase of the amount of light emission occurs and the light emission gradually increases with a time lapse even during the non-emitting period in response to the increase of the amount of the captured electrons and holes, as shown in FIG. 7B. With the constant-rate injection, the number of carriers in the active layer 1 gradually decreases during the emitting period, resulting in a gradual reduction of the radiated output in the emitting period.
In short, with the above light emitting semi-conductor device of the voltage control type, as long as the injection rate of the electrons and holes is constant, the steady state amount of radiated light settles at a certain level both in the emitting period and in the non-emitting period, which certain level depends on the constant injection rate. Accordingly, the amount of the radiated light in the steady state becomes constant regardless of the bias voltages at the control electrodes 4 and 5 of FIG. 6. Besides, if modulating pulse voltages with a short pulse duration are applied successively, the amount of the electrons and holes in the active layer decreases gradually, resulting in a gradual reduction of the level of the radiated output. Further, the injection efficiency of the arrangement of FIG. 6, i.e., the injection of electrons and holes from opposite sides of the active layer, is rather low.